


Blue Balls

by boneapptheteeth



Category: Undertale (Video Game)
Genre: Accidental Voyeurism, Alternate Universe - Underswap (Undertale), DFAB reader, Dubious Consent, F/M, Possessive Behavior, Smut, Underswap Papyrus (Undertale), Underswap Sans (Undertale), like borderline yandere, very very intense sans
Language: English
Status: Completed
Published: 2019-04-04
Updated: 2019-04-04
Packaged: 2020-01-04 23:23:15
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 5,120
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/18353843
Author URL: https://archiveofourown.org/users/boneapptheteeth/pseuds/boneapptheteeth
Summary: Your whole relationship with Sans seemed to be lead by the skeleton himself, and you were just along for the ride. You really didn't mind, you were content to let the energetic monster do his thing. Your relationship kept developing steadily, though you could never bring up something like sex with the cute skeleton - after all, he was just too innocent to taint, wasn't he?Looks like you were going to find out just how wrong you were.





	Blue Balls

**Author's Note:**

> ahh.. um. i can explain. i just really wanted to write actual action and so skull and crossbones and this was made. i swear i'll go back to my main fic.

You didn’t know when this all became normal – living in a house inhabited by skeletons that is.

 

You had started living with the skeleton brothers due to unforeseen circumstances soon after you had befriended them. Before the brothers, your life seemed somewhat normal and drab, you being the kind of person that travelled safer paths and rarely took risks. It was only till you met Sans that excitement blossomed in your days and passion took root in your heart.

 

The short skeleton was excitable to say the least. He was happiness and thrill wrapped in a big, blue bow. His loud voice made up for his smaller stature, and his chubby cheeks only proved to make him look absolutely adorable. Due to this, he was often mistaken as a child; even you had thought him to be much younger than he was upon your first meeting. This was quickly remedied by his deeper voice and his younger brother’s guffaws of amusement. That’s right, _younger_. Sans was the older brother of the two.

 

His younger brother, Papyrus, was a tall and lanky monster, his laid-back and almost uncaring nature directly contrasting his brother’s boisterous one. The two made quite a sight, Sans would lecture him about something the taller skeleton did just to tease his brother, and Papyrus would recite a bunch of puns – grin growing at every exasperated groan ripped from Sans.

 

It was honestly precious how Sans has raged at the misunderstanding involving his age, shouting at his brother to stop laughing. He somehow twisted the situation to be the other skeleton’s fault, apologising to you for his brother’s insolence. You would have been worried about their dynamic as siblings, had you not seen the smile threatening to break Sans’ otherwise angry expression and the sparkle in Papyrus’ eye… socket. Your heart melted at the two. This was what lead you to exchange numbers with the boys, promising to keep in-touch. Though, your hopes weren’t high since your conversations usually died with people when you needed to keep in contact through text.

 

To your surprise, Sans was the one who pushed for consistent conversation; be it talking about his day, asking about your day, or sending you a funny meme he found online. You quickly grew fond of Sans – it really was inevitable due to how freaking adorable he was – and he became a treasured friend in no time. You were less close with Papyrus, but you both didn’t mind being around each other. In fact, there were many nights where you would eat dinner over at their place and the two of you would share laughs while Sans was occupied in the kitchen. Sometimes you would catch Sans pouting at you two before catching your gaze and giving you that 100-watt grin that you couldn’t resist. So, obviously, you shrugged off the oddness of those moments and continued your fun with the skeleton brothers.

 

When you moved into the brothers’ house, your life changed once again. Thing time it was because you couldn’t seem to detach Sans from your person when you were both home. This consisted of lots of snuggling on the couch, his arm latched onto yours and his head on your shoulder when you were working on your laptop, and his arms around your torso while his face was stuffed into your back when you were grabbing a snack from the kitchen. You really didn’t mind too much – after all, you could appreciate a good cuddle from time-to-time – but sometimes it really got in the way of certain things such as when you wanted to leave the house or use the washroom. Sans would always whine and send you that heart-breaking stare when you shifted to leave his grasp, making you succumb to his adorable ways more than once. You almost peed your pants once because you didn’t want to upset the small skeleton.

 

One day, Sans had confessed to you. This was… odd to you because you hadn’t even considered dating the monster. It wasn’t _because_ he was a monster, but it was more like he was a friend you wanted to protect no matter what. You didn’t want to consider that the cause of your hesitation was because you thought of him like a child. That was… insulting to him to say the least. You of all people should have known that Sans was by no means a child. This was not only because of his age, but because he had proven multiple times that he was not innocent nor naïve, and he was quite cunning and scheming. For god sakes, his hobby was making puzzles; though these weren’t jigsaw puzzles, no, he made complex, brain-melting puzzles that you couldn’t figure out for the life of you. The one time you actually solved one of his puzzles you had been overcome with joy, Sans having had joined your joy as the two of you jumped around the living room with Papyrus chuckling to the side at your antics. Sans was a fully-functioning adult with a child-like joy deeply embedded in his heart (though you prevented yourself from thinking about the impossibility of a skeleton having a heart), and you could admire the compassion and tolerance that must take of him.

 

You felt it wrong to accept Sans’ confession when you weren’t sure you felt the same, but upon further contemplation and the pressure of the skeleton’s pleading look, you relented. You made it abundantly clear to Sans that you didn’t feel the same way as him (his sad look would haunt you till you died, and then some), but you told him you were willing to try it out and see where it went from there. You let him celebrate before you told him that he needed to let you go if you decided it wasn’t working out and he responded with an ‘OF COURSE!’ that somehow… sounded insincere, but you were willing to give the cute skeleton the benefit of the doubt.

 

After you started dating, well, he started getting even more clingy – you never thought it was possible. Sans always seemed to have a massive grin around you, nuzzling his face into you any chance he got, and running his hands over the surface of your skin constantly. He even had begun to sit in your lap when you watched movies. Papyrus, on more than one occasion, had commented on how ‘sickeningly sweet you two are, can you tone it down for my sake? kinda gross to see my own brother like that.’. Sans refused, and you had only sputtered in embarrassment and hid your face, the short monster refusing your attempts to escape. Papyrus would usually proceed to fake gag and walk out of the room and his brother would only snuggle closer.

 

The only times you seemed to get some semblance of peace was when Sans left the house for his early morning training or for work, though, even then you were barely awake during the times he worked out and you had work during similar times that he did. You were so glad you had managed to convince the monster not to drag you out of bed to join him in his training. You may have liked a good workout occasionally, but you knew how intense the skeleton got and would rather not die of physical exhaustion, no thank you.

 

It surprised you immensely when you realised you had started… feeling things for Sans. To be honest, despite the fact that you had given him a chance, you had completely expected for nothing to come of the attempt. You had never really held any sort of romantic affection for anyone before, and the people who had been interested in you previously always lost interest when they saw how detached you were. Whenever you saw Sans, something warm blossomed in your chest, and when he cuddled into your side you couldn’t stop yourself from nudging yourself closer. But despite this, you could never begin to consider doing something like having sex with him. Even thinking about that… made you feel disgusting. How could you even consider defiling the cute skeleton? You were sure that he was asexual – though you knew you should probably ask him rather than assume, you could never start that conversation and risk his discomfort. You rather liked the mood you two built in the last while; ruining that would just about break your heart. Unfortunately, this meant you had a lot of sexual frustration built up from your inability to leave Sans’ side. You took every chance you got to burn some of it off the preserve your cozy relationship.

 

* * *

 

Sans was incredibly impatient, and that was saying something for a patient soul like his.

 

He followed all the steps in his dating manual! He gained your friendship, courted you, he asked you out, and he stayed as close to you as he should! Okay, maybe the dating manual didn’t say anything about constantly rubbing his scent on you, groping your squishy human flesh whenever he got the chance, or ogling your beautiful form – but he hardly thought it would make a difference! He even kissed you, though maybe you hadn’t realised he was doing so since the closest equivalent was nuzzling his teeth into you. But still, he was pent up and all he wanted to do was pin you down and… no. He refused to get a hard-on randomly like that by thinking about what he wanted to do to you… again. It was unbearable how sexy you were; you had to know what you were doing to him! Though, despite this, you continued sending him those cute smiles and patting his head like he was an adolescent.

 

He wasn’t a child!

 

When he first met you, you had been a strong candidate for a potential friend. He was over the moon when you had exchanged phone numbers, and tried his best to keep in contact less he wasted an opportunity to gain the admiration and camaraderie of another. He was slowly increasing his number of human friends, and you were the newest addition! Maybe even better than the other friends he made since you were so kind and cute. Those thoughts kept growing and developing as he texted you and invited you over, your adorable personality quickly enamoring him. You were down-to-earth, enthusiastic to try new things, and put your heart into everything you did – clearly a magnificent datemate to have! He didn’t want to say he was happy that you had to leave your home… but how could he deny the spark of pure elation in his soul when you accepted his offer to stay with him and his brother?

 

That was another thing… Papyrus.

 

Having another male so close to you irritated the heck out of him; he couldn’t stand it when he caught you two in a fit of laughter, shoulders brushing against each other like you were in some sort of romance movie. You were _his_ datemate!

 

Well, you hadn’t agreed to it at that point, but he already considered you his. If you ended up denying him, well, he’d find a way.

 

It wasn’t as if you weren’t around potential rivals constantly, but Sans knew that you wouldn’t bat an eye at any of the men or women at your workplace or on the street – you had the best taste after all! You deserved better and you knew it. Luckily, _he_ was the best!

 

So, to solve the issue with his brother, Sans gave Papyrus a stern warning and lathered his scent on you any chance he got. The best thing was that you didn’t seem to mind! You were practically Sans’ datemate already, you just had to affirm it verbally and you’d be all his!

 

And that was what he did next, he went to you to gain your spoken word. Though, he wasn’t expecting the near rejection he got. It seemed that the only thing that saved him was your kindness and his irresistible charms. Okay, so maybe he manipulated you a little bit into doing what he wanted, but he couldn’t help that you were susceptible to his natural skeletal wiles and big starry eyes. You were putty in his hands and that’s just how he liked you; ready and willing for him to do what he wanted with your gorgeous mind and body. Mweh!

 

With the greenlight, Sans put even more effort into cuddling into your form, high off the permission you gave him to be more than friends with such a lovely creature. He dropped as many hints as he could, nudging you into taking action to move on to the next step in your relationship, but you seemed blissfully oblivious, always taking his comments and groping in stride and laughing off his suggestive jokes. He found it adorable but frustrating all the same, why couldn’t you just let him rut you into his mattress already?!

 

Despite these thoughts, your wants were much higher on his list of priorities than his raging libido, so he let you be until you told him you were ready. He could endure how the shape of your backside ignited a flame of burning passion in him in favour of violating your trust.

 

Though certain moments made the wait unbearable.

 

One night, Sans nearly snapped and took his datemate right in front a room full of people. He had been with you and his brother at Muffet’s, and though he normally didn’t condone that grease trap, you had begged him to take you since he hadn’t let Papyrus do so before; for obvious reasons. It seemed as though Papyrus’ constant bragging about the establishment finally got to you, causing you to desperately want to try out the place for yourself. And being the magnificent and understand datemate that he was, Sans agreed to allow you to poison your body with Muffet’s unhealthy consumables this one time. Any other attempt to coerce him would fall through – you needed to maintain a healthy lifestyle after all.

 

So, the three of you decided to have dinner at Muffet’s one night. Everything was going well, you were beaming with happiness at the atmosphere and the food (though he would disagree with your apparent enjoyment of the grease-filled fries, then again, he couldn’t outright say that it was disgusting… but his tacos were still many leagues above the food there!) and his brother seemed more relaxed than he had been for a long time. The happiness of the two most important people made Sans’ own happiness skyrocket. It went downhill when you got up to refill your drink at the bar.

 

With a smile, you announced that you were going to head to Muffet to ask for a refill and that you would be back. The two skeletons affirmed that they would be waiting and you headed off. He should have gone with you, or at least headed your way the minute he realised you had been gone too long. As soon as Sans started getting suspicious, he looked your way to see the creepy red bird monster who was always at the bar whenever he had to pick up Papyrus with a wing on your arm and his face much, much too close to yours for Sans’ liking. Filled with rage and pure jealousy, Sans sped his way to the two of you and ripped the slimy limb off of yours, a low growl rolling out of him as a warning. The skeleton had half a mind to kill the other monster right there and claim you in front of all the other patrons on the bar top to make a point (monsters were usually pretty territorial, and though that was still a little over the top, he was sure the monsters in the room would understand to some degree why he did what he did), but his brother’s restraining hand on his shoulder stopped him, and his scolding glance at the shorter skeleton calmed him enough to allow him to think rationally. You were safe, and his, and you looked confused as hell instead of scared so that was a plus.

 

Sans took a calming breath and put an arm around your waist, asking you if were okay and suggesting the three of you take the rest of the food for take-out. After Papyrus agreed, your hesitancy became a confused shrug and the group headed out soon after.

 

That incident was the catalyst to Sans’ more territorial behaviour. He soaked more of his scent into you before you left, and usually followed you to your destination to assure your safety before he continued on with his day. He slept in the same bed as you every night and gave you skeleton kisses constantly. You seemed to be starting to get suspicious and wary of his changed behaviour, but he always convinced you that everything was fine and that he just loved being around you, which was true, but it was also because monsters tended to move faster in their relationships. With the confidence Sans had in his relationship with you thus far, you should have been talking about soulbonding already, yet he knew that it took up to a few years for human couples to begin to talk about spending the rest of their lives together. He couldn’t wait that long! He already knew he would want to grow old with you and dust by your side.

 

Maybe he should drop clues around for you, like having you find a conveniently placed book on souls and bonding, or maybe he could ask Asgore to bring up the topic the next time you two met.

 

He was still thinking about what to do when something happened that… made all of his scheming unnecessary.

 

On Thursdays, Sans was usually kept at work a bit later than his other shifts, but on that particular day his boss decided that there wasn’t too much work to do and let him go earlier than usual. This was why he arrived home sooner than he typically would, excited to spend more time with his datemate; especially because he knew his brother wouldn’t be back yet, so the two had the house to themselves.

 

The monster announced his arrival by word as well as by the slamming of the door – what could he say, he loved to make a dramatic entrance, and exit, and existence in general.

 

He scrunched his face up curiously when he didn’t receive a response, that was unusual, but there were occasional times when you listened to music much too loud than was good for your ears through your earbuds. Sans dismissed the oddity of your lack of response as a result of his unexpectedly early arrival. He just had to surprise you in your room! Your alarmed squeak was always so adorable, he never got many chances to surprise you like that so he would take the opportunity without question.

 

Climbing up the stairs, he realised there was a… weird scent in the air. It wasn’t a bad weird, it was a very, very _good_ weird. The kind of weird that wrapped around his bones and made them heat up, the kind of weird that filled his skull up with dirty thoughts about how he wanted to rip off all your clothes and bury his skull into your cunt, filling you with his tongue until your throat was sore from screaming. He’d bring you to a climax and fill your oversensitive core up again, but this time with his cock. He’d chuckle at your begs for mercy and-

 

Wait, what? Why was thinking about this all of a sudden?

 

He was just thinking about that scent… the scent that smelled musty, and delicious, and strangely like you. In fact, it smelled just like a super condensed version of your natural scent.

 

He had an inkling of what it was, he just had to… confirm.

 

Sans reached the top of the stairs and quickly made it to the front of your door. It was ajar, as it usually was, and before he could lift a hand to open it, something made him freeze.

 

Specifically, you made him freeze.

 

He heard a whimper from within the room, the kind of whimper that set his pelvis aflame and had his soul tugging to leave his chest. He suddenly registered the faint sound of inconsistent buzzing.

 

Sans gulped when he heard you speak,

 

“Y-Yeah…! That feels-!” You gasped.

 

The skeleton was now trembling with the effort to hold himself back. He just wanted to rip the door open and push you against a wall in your room, lifting you against his form so that you couldn’t escape when he slammed his aching dick into your soft velvety heat. You’d be so tight, squeezing all around his shaft and massaging him to completion. He’d play with your throbbing clit until you whined that you couldn’t cum anymore, and he’d push you until you really couldn’t cum any longer. Sans would leave bruises and bite marks all over your beautiful needy body, and you’d be caught between begging him to stop and pleading him to never stop. He could rut into you forever, lick up all your juices for eternity, and pleasure you until you’d never want to leave his side. You’d be sore for _weeks_.

 

But he couldn’t! He needed to wait for you to tell him you wanted all that, needed to make sure you understood how badly he needed you. Though, he wasn’t able to will his body away from your delicious sounds.

 

In fact, deep in his thoughts of making you his, Sans hadn’t noticed when he stepped forward and pushed the door open.

 

He did notice, however, when he caught sight of your absolutely maddeningly delicious-looking body.

 

You lay on your back, mattress bearing the weight of your squirming self. Your shirt was pushed up above your breasts, nipples red as if you had been toying with them repeatedly, and your lower half bare. You listened to music, as he suspected earlier, on a set of earbuds connected to the head that was thrown back. Back arched, one hand held a blue object that you pushed in and out of your soaking pussy at a brutal pace, and the other roughly circled around your clit. The wet sounds of your heat would forever haunt his dreams.

 

His mouth watered when your spine arched even further and your face turned into the pillow below you head. Your eyes were clenched shut in pleasure.

 

He swore he actually drooled when he heard you spoke next, his hand lifting to grope at his twitching length that pushed uncomfortably against his pants without his permission.

 

“Ah…! Oh g-god, I’m gonnAH!” You spread your legs further and pushed the vibrator into yourself harder, moaning and squeaking at the increased stimulation.

 

Sans couldn’t… he couldn’t. He prided himself as being a strong-willed and patient monster. But this, _this_ was way too much. The glance of your pink glistening pussy was enough to break him, but what came next was really the thing that made him lose all semblance of control.

 

“Yes! P-Pleaseeee, harder! S-SAAANS!” You were thinking about him. You were thinking about him fucking you while you masturbated. You wanted him to fuck you as much as he wanted to fuck you, wanted him to fill you with his seed until you were dripping with his scent, until you were full and sure to have his child. You would be so sexy with a large belly, nursing his child he made with you.

 

He growled loudly, unable to hold himself any longer.

 

Sans pounced.

 

You screamed when the skeleton jumped on top of you and pinned your wrists up beside your head. You still panted when he settled on top of you, eye sockets dark and fangs bared; you didn’t even know he had fangs. Sans used his teeth to remove your earbuds.

 

“Sans? W-What-?” You stuttered out before he interrupted you in a low register.

 

“Human. How Could You Keep That Sweet-Looking Pussy From Me For So Long If You Wanted Me To Fuck It So Bad, Huh? You’ve Been A Bad Girl, Haven’t You?” You gulped, eyes wide in disbelief. Sans could smell you getting wetter with each word. He smirked.

 

“Oh, What A Naughty Human. You’re Enjoying This, I Can Tell.”

 

He shifted so both of your wrists were in one of his, not that you were struggling, and lowered a bony hand to where the blue vibrator was still sheathed in you. The toy came out with a lewd squelch, the sensation making you shiver. Sans brought up the silicone object for inspection, sniffing it and then running his magic tongue up the side of it. He groaned at the taste, and you whimpered.

 

“You’re Just As Delicious As I Imagined, I Can’t Wait To Eat From The Source.” Sans then glared at your sex toy and threw it aside with a growl.

 

You finally were able to shake yourself from your shocked silence, “W-What are you doing, Sans? Why…” you trailed off, unable to even begin to question what was happening.

 

Sans chuckled, “Don’t Worry My Darling, You Won’t Be Able To Think Soon.” He brought a phalange to your clit, rubbing it briefly and drinking in your consequent exclamation and the violent twitch of your hips, Sans groaned and ground into your leg, “Regretfully, I Don’t Think I Can Feast from your Beautiful Core – I Can’t Wait Long Enough To Sink Into Your Beautiful Body. Luckily, You’ve Already Prepared Yourself For Me, Though I’ll Have to Punish You For That Later. But That Can wait, I’ve Waited Much Too Long For This.”

He used some of his magic to continue pinning your wrists above your head, using his free hands to undo his pants clumsily.

 

“W-Wait! I’m not sure about this.” you almost forgot what you were saying when Sans’ thick blue cock was released from its constraint, the glowing appendage oddly captivating for what it was.

 

“You Want This Right? You Want Me To Fill You Up To The Brim With My Dick, And Bring You The Greatest Pleasure That You’ve Ever Known?” Biting your lip, you continued to stare at the very dick he was talking about.

 

“Yes, b-but-!” He growled.

 

“Then I’m Going To Do just That And More. I’m Going To Fuck You So Hard That You Won’t Be Able to Leave This Bed Because You’ll Be So Sore, And the Only Thing You’ll Be Able To Think For Hours Is Me, You’re _Mine_.” He positioned his member at your entrance, the slickness of you easily allowing him entrance. He thrusted his hips slightly and his head entered a bit before he held back. You subconsciously wrapped your legs around his pelvis and pulled him closer.

 

Sans buried his face into the crook of your neck, taking a deep whiff of your irresistible scent, “You’re Mine, And No One Can Take You Away from Me, You’re _My_ Mate.” And with that, he plunged into you, teeth flexing into your shoulder. You cried out in pain and pleasure, fingers curling with the desire to grab onto your lover.

 

Your skeletal boyfriend groped a breast as he vigorously pounded his growing cock into you, your dripping pussy sucking him in further. His thrusts were uncoordinated and inexperienced, but what he lacked in technique, he more than made up for in raw power. The thickness of his cock stretching you out deliciously. You moaned his name as he moved to your other shoulder and proceeded to leave another mark on your skin, blood pooling down onto your sheets.

 

“You Feel So Good, Mate.” He groaned out, “Mine To Take, To Fuck. You’ve Been Craving My Cock Haven’t You? How Does It Feel?” he paused to nuzzle into you, “Your Cunt Is Everything I Imagined And Much, Much More.” He moved to play with your clit, you screamed in response, clenching around him and arching your back.

 

“You’re So Beautiful, I Just Want to Do This With You Forever. I Want to Pull Out Your Soul And Bond With You Right Here; Cum In You So That You’ll Get Pregnant From The Thorough Fucking I’ve Given You.” Your hips twitched to meet his at that and he rested his forehead on your shoulder, “Yeah, You Like That. You Want Me To Fuck You So Hard That You’ll Carry My Child? I’ll Spill In You So Much That They’ll Be No Way That You’re Not With Child. I’ll- _Fuuuuck_.”

 

You had shifted so that you could suck on his clavicle, licking more when he began to move his hips harder and growl. Sans nudged you away in favour of latching his teeth around your nipple and lathing it with his tongue. You couldn’t help but make a high-pitched sound of pleasure, feeling your climax approaching.

 

“FFFuck!” his growl was intense and he bit the side of your breast before lifting his head and meeting your eyes, “Cum For Me, My Mate. Show Me Your Body Is Mine. You’re _Mine_. **_Mine_**!” he slammed his teeth into your lips and tangled his tongue with yours and his thrusts became desperate.

 

You tilted your head back and as a result your kiss was broken, a string of saliva connected your mouths and snapped. You screamed his name in the wave of your building orgasm, your legs tightening around him to an almost painful degree and toes curling.

 

“Yes! Scream Only My Name!” when his phalange on your clit harshened its rubbing, you were tipped over the edge. You couldn’t even comprehend what was leaving your mouth in the height of your pleasure, the world around you flashed white. Sans followed you soon after, he could only fit in a few more hard thrusts before the grip of your cunt became too much for him to handle and he let out his load, shallowly pumping into you until he was finished.

 

Sans rested over you, arm still supporting his weight on the bed. You both panted, coming down from your highs.

 

“That was… amazing.” You said as you gulped in as much air as possible.

 

He only smirked, lifting one of your legs over his shoulder,

 

“Who Said We Were Done?”

 

Sans made good on his promise to fill you up that night, after hours of endless sex. He eventually did explain soulbonding with you afterwards, and though you were hesitant about the amount of commitment it required, you agreed to give it some serious thought. You thought you were seriously coming to love your skeletal lover.

**Author's Note:**

> i feel like this didn't flow as well as i wanted it to? well, whatever. 
> 
> i'm not sure why i decided to write this actually? i usually like to write for the skeletons that adopt the original sans' traits. but here it is, i guess. i headcanon swap sans to be a patient soul and swap papy to be a brave soul just like their tale counterparts, i feel like the papyruses would also be more calm and subdued when it came to this stuff and the sanses would be... very enthusiastic. mmm just my opinion though. 
> 
> also sans definitely learned what he knew about sex from porn. no take backsies. 
> 
> visit my tumblr!  
> https://bone-app-the-tea.tumblr.com/


End file.
